Shock absorbers for the wheel suspension of motor vehicles, in which the damping force may be varied, have already been available for a long time. For this purpose, various approaches are known from prior art. On the one hand, prior art suggests shock absorbers fitted with continuously variable valves and which allow for the damping force of the shock absorber to be varied continuously. The implementation of such continuously adjustable valves implies some technical effort so that such shock absorbers are comparatively expensive to manufacture.
Besides shock absorbers with continuously adjustable damping force, also known from prior art are shock absorbers wherein a defined arrangement of passive damping valves in conjunction with a control valve allows for setting different damping characteristics. In particular, it is possible to set different damping characteristics for the rebound stage and the compression stage. An example of this kind of motor vehicle shock absorbers with variable damping force is described in document DE 38 03 888 C2.
From document DE 35 18 327 (U.S. Pat. No. 4,723,640), a hydraulic adjustable vibration absorber is known. In the shock absorber known from this document, a damping piston is provided that is provided on a pin of a housing screwed onto the piston rod. In DE 35 18 327, this housing is designated by the term “intermediate sleeve”.
The damping piston has damping valves cooperating with flow passages provided inside the body of the damping piston. Such damping valves are configured so as to have a hard damping characteristic, i.e. so as to produce high damping forces when damping liquid flows through the flow passages. A bypass is provided hydraulically in parallel to the damping piston. Inside the bypass, a bypass valve is provided. The latter has a soft damping characteristic, i.e. when damping liquid flows through the bypass valve, only a relatively low damping force is generated in comparison with the damping piston.
The bypass valve is provided on a flow path formed through a transverse bore in the housing, i.e. in the envelope of the “intermediate sleeve”, and a longitudinal bore in the pin supporting the damping piston. Inside the housing, a control valve is provided. The control valve comprises a coil and a valve body made as an armature. When the coil is not energized then the valve body is seated on a valve face, not identified by a reference number in DE 35 18 327, of a component having an H-shaped cross-section, also not designated more in detail. In this state, the control valve is closed (“closed when currentless”). When the coil is energized, the valve body lifts from the surface against the force of the return spring. The damping medium may then flow from the lower working chamber of the shock absorber into the upper working chamber or in the opposite direction through the bypass. Here, the damping liquid flows through the bypass valve having the soft damping characteristic, so that the shock absorber only generates comparatively low damping forces.
In the closed state of the control valve, however, the bypass is closed so that there is no flow through the bypass valve. The damping liquid then flows through the flow passages in the damping piston and cooperates with the damping valves provided on the damping piston and that have the hard damping characteristic. Due to this arrangement, the safety-relevant demand of a so-called “fail-safe function” is satisfied. “Fail-safe” means that in case of sudden failure of the electrical power supply of the shock absorber, the damper must generate a hard damping characteristic in order to warrant safe driving stability even at high speeds and e.g. when cornering.
In the design known from DE 35 18 327, the disadvantage is that the valve arrangement disclosed therein is only suitable in a limited way for an implementation in shock absorbers having a small damper tube diameter. The damping valve having the soft damping characteristic ensures comfortable damping, providing the passengers of the vehicle with a pleasant driving experience. Therefore, the person skilled in the art also designates this valve as a “comfort valve”. Due to the soft plating of the comfort valve, less resistance is opposed to the damping liquid, and thus smaller damping forces are generated. In order to be able to provide a sufficiently large flow section for the soft damping characteristic, even at higher damper speeds, a diameter as large as possible for the comfort valve and the valve disks thereof is targeted. However, in the shock absorbers described in DE 35 18 327 A1, for design reasons, the comfort valve only has a relatively small diameter. Consequently, at least when the damper tube only has a small diameter, it is not possible to design this comfort valve to be large enough to offer sufficient driving comfort and spread between the hard and soft damping characteristics, even at higher damper speeds. Also, the durability of smaller diameter valve disks is significantly lower than for larger diameter valve disks. Moreover, the valve arrangement described in DE 35 18 327 has a large overall axial length due to the cross-section of the H-shaped element on which the armature/valve body of the control valve is sealingly resting.
Another disadvantage of the subject matter of DE 35 18 327 is that, as seen in the axial direction, the damping piston must have a minimum overall height if the shock absorber is to be used in a McPherson strut unit. Indeed, in such strut units, the steering movements of the vehicle are introduced via the piston rod of the shock absorber so that the corresponding transverse forces are transmitted via the damping piston to the damper tube. This can no longer be ensured if the damping piston falls short of the minimum overall height. Therefore, in DE 35 18 327 A1, the possibilities for reducing the overall axial height of the damping piston used for the hard damping characteristic are quite limited.
In DE 40 08 326, a hydraulic, controllable vibration absorber is described in which at least three different damping force levels can be set by an electromagnetically activated valve. A flow passage is controlled by the electromagnetic valve. In order to generate three different damping force characteristic curves, the valve body can be moved between two end positions, wherein the valve body is supported by at least two springs and can be set between the two end positions thereof in at least one intermediate position via a stop and a stop face. In the respective position, the valve body switches a corresponding bypass so that depending on the position of the valve body, a hard damping characteristic, a soft damping characteristic, and an intermediate average damping characteristic is set. Both springs are opposed to the magnetic force so that in the currentless state of the solenoid, both springs move the valve body into the normal position thereof. When the solenoid has been energized for the first time, the valve body is brought to a stop against the first spring, so that in this position, a bypass is switched. In a consecutive larger energization of the solenoid, the second spring is biased so that the valve body can be brought into the end position thereof, with a second bypass being released in this end position.
In the solution described in DE 40 08 326 C1, it is necessary to differentiate energization of the solenoid so that the valve body can be switched to at least three positions including a starting position. Consequently, it is either necessary to control current intensity when energizing the solenoid, or else—as claimed in dependent claim 8 of DE 40 08 326 C1—the solenoid must have at least two different magnetizing coils. Both of the above-mentioned options are in contradiction with a cost-effective shock absorber design.
Furthermore, the constructional configuration of the control valve according to DE 40 08 326 C1 is elaborate and cost-intensive due to the required springs for supporting the valve body and due to the required stop and stop face.
Moreover, in the solution disclosed in DE 40 08 326 C1, the hard damping characteristic is not provided simply by a single damping valve having a hard damping characteristic, but a sporty hard setting of the vibration damper is achieved by an elaborate hydraulic series connection of the valve disk sets called “spring leaves 16, 17”, cf. FIGS. 1 and 2 as well as col. 3, lines 11-14.
Finally, with reference to the solution disclosed in DE 40 08 326 C1, it is disadvantageous that the rise, i.e. the gradient of the damping characteristic curves in the force/speed diagram is fixedly set in the range of low piston rod speeds. Consequently, in the range of low piston rod speeds, this solution does not meet the demand of flexible adaptability of the gradients of the damping characteristic curves to different customer demands.